This invention relates to a magnetic recording medium having a magnetic recording layer of ferromagnetic metal film, and more particularly to an improved magnetic recording medium of the character having high output and high C/N characteristics and, moreover, increased durability.
Technology of magnetic recording media using a Co-Ni, Co-Cr, or other ferromagnetic metal film as the magnetic recording layer has been widely studied and carried into practice. It is aimed primarily at higher density recording in video, digital, and other recording techniques and at the introduction of smaller, higher-performance recording-reproducing devices than heretofore.
To enhance the magnetic characteristics of those media for services over broader bandwidths than heretofore, attempts are being made to provide magnetic layers of multiple structures. For example, Japanese Patent Application Publication No. 21966/1991 discloses a magnetic recording medium having a magnetic layer fabricated from multiple layers of a ferromagnetic metal by vacuum vapor deposition, ion plating, sputtering, or other technique. The multilayer construction has dual purposes of reducing noise and allowing an under-layer ferromagnetic metal film to secure an output on the low frequency side with a low coercive force and an upper-layer ferromagnetic metal film to secure an output on the high frequency side with a high coercive force.
There is shown in FIG. 1 an exemplary magnetic recording medium of the prior art having multiple layers of a ferromagnetic metal film. Over the surface of a nonmagnetic base, e.g., of polyester are formed two or more magnetic layers, e.g., under-layer ferromagnetic metal films 4, 4' and an upper-layer ferromagnetic metal film 6. Generally, the vapor deposition angles of the individual ferromagnetic metal films (the deposition angles as measured from lines normal to the film surface on the nonmagnetic base, the same applying hereinafter) are made larger in the lower layers and smaller in the upper layers. It is also common that the deposition angle of at least one of the under-layer ferromagnetic metal films 4, 4', e.g., the metal film 4', is made reverse to the deposition angle of the upper-layer ferromagnetic metal protective film 6, so that, as viewed from the magnetic head, a generally low coercive force is maintained to secure a low-frequency output whereas the upper-layer ferromagnetic metal film 6, as viewed from the magnetic head, generally maintains a high coercive force to secure a high-frequency output.
Ferromagnetic metal films usually are worn rather easily by friction with magnetic heads and other sliding members. It has therefore been proposed to apply a lubricant or form a lubricant layer on the ferromagnetic metal film surface to reduce the friction, or to provide a sufficiently hard protective film to increase the wear resistance. For example, Japanese Patent Application Public Disclosure Nos. 132623/1990 and 224132/1991 suggest improvements in the durability and running stability of ferromagnetic metal films by the formation thereon of a diamond-like carbon protective film and by the further formation of a lubricant layer. The diamond-like carbon protective film, which is only slightly permeable to moisture, has an anticorrosive effect and thereby helps maintain the magnetic recording characteristics.
Nevertheless, the recent tendency is toward higher density digital recording on magnetic recording media, and those media of multilayer structures described above are unable to meet the ever growing high output requirements.
It is therefore an object of the present invention to provide a magnetic recording medium capable of higher density recording than before.
The magnetic recording medium of the type comprising a nonmagnetic base of polyethylene terephthalate or other plastic, a ferromagnetic metal layer formed thereon, and a diamond-like carbon protective film further formed thereon has a problem of magnetic characteristic deterioration. This is because the nonmagnetic layer permits the permeation of moisture to some extent and is thereby corrosively attacked from the underside upwardly with extended use. The problem is particularly serious on the high frequency side. Another problem is curling that results from the formation of the ferromagnetic metal film on a plastic surface by vapor deposition, sputtering or the like, with the metal film embraced by the substrate. Formation of the diamond-like carbon protective film on the metal film curls reversely, thus tending to offset the curl with the ferromagnetic metal film. However, unless the two films are properly combined, the curl compensation is inadequate and makes the contact of the medium with a magnetic head incomplete, failing to keep the magnetic recording characteristics constant.
Therefore, it is another object of the invention to provide a magnetic recording medium of a high-output, high-C/N ferromagnetic metal type having a satisfactory anticorrosive effect and a limited curling tendency.